Methods for producing alcohols comprising carrying out reduction reaction of starting materials of an alcohol, such as a fatty acid ester, a fatty acid triglyceride, and a fatty acid, in the presence of a hydrogenation catalyst have been already known in the field of art. The reduction reaction is generally carried out under conditions of a pressure of from 200 to 300 atm. and a temperature of from 200 to 300.degree. C. In view of carrying out such methods in an actual production plant, however, a process at such a high pressure requires not only equipments with sufficient pressure resistance but also sufficient maintenance of the equipments for keeping air-tightness and the like. Such equipments would require high equipment investments and high running costs.
In order to overcome these problems and produce alcohols at lower costs, various studies have been recently made on hydrogenation reaction at a relatively low pressure. The results of the studies are disclosed in, for example, Japanese Patent Examined Publication No. 4-72810, Japanese Patent Laid-Open No. 64-47726, Japanese Patent Unexamined Publication No. 4-504408 and Japanese Patent Examined Publication No. 4-57655.
Japanese Patent Examined Publication No. 4-72810 discloses a method for producing a fatty acid alcohol comprising hydrogenating a fatty acid having a number of carbon atoms corresponding to the resulting alcohol, or an ester thereof at a reaction pressure of from 20 to 100 bar and a reaction temperature of from 150 to 300.degree. C. Similarly, Japanese Patent Laid-Open No. 64-47726 discloses a method for hydrogenating a fatty acid methyl ester in a molar ratio of hydrogen to ester of from 10:1 to 500:1 at a reaction pressure of from 20 to 100 bar and a reaction temperature of 160 to 270.degree. C.
Under the above-mentioned conditions, a fatty acid or an ester thereof remaining as unreacted starting materials and a wax ester formed as a reaction intermediate are included in the resulting alcohol at the outlet of the reactor, and hence, the yield of alcohol is liable to be lowered. The reasons for a low yield of alcohol are as follows: In the hydrogenation reaction, the amount of hydrogen dissolved in a liquid phase comprising a fatty acid or an ester thereof in the reaction system is decreased as the reaction pressure is decreased, thereby drastically lowering the reaction activity. In addition, the chemical reaction for producing an alcohol is an equilibrium reaction, and the equilibrium shifts to the side where a wax ester formed as the reaction intermediate remains in the resulting alcohol at the given pressure conditions mentioned above. Furthermore, the resulting alcohol is also reduced to hydrocarbons under the above-mentioned conditions, which leads to a further decrease in the yield of the alcohol. Additionally, in the case where a fatty acid having 8 to 18 carbon atoms or an ester thereof is hydrogenated under the above-mentioned conditions, the boiling point region of a hydrocarbon formed as a by-product overlaps with that of a short chain aliphatic alcohol, and hence, it is difficult to separate the hydrocarbons from the alcohol in the subsequent processes by distillation. Accordingly, in order to carry out hydrogenation reaction under the above-mentioned conditions, it is necessary to separate a short chain fraction from a long chain fraction in a fatty acid or an ester thereof before the hydrogenation reaction.
Japanese Patent Unexamined Publication No. 4-504408 discloses a method for producing an aliphatic alcohol comprising hydrogenating in a gas phase reaction a lower alkyl ester of a fatty acid under the conditions of a reaction pressure of from 5 to 100 bar, a reaction temperature of from 140 to 240.degree. C., and a molar ratio of hydrogen molecules to ester of from 200:1 to 2000:1, wherein a mixed material of the ester and hydrogen molecules is brought into contact with a hydrogenation catalyst at a temperature constantly kept higher than its dew point.
By conducting the hydrogenation reaction in the gas phase under the above-mentioned conditions, the conventional problem regarding the amount of hydrogen molecules dissolved in the liquid phase at a low pressure can be solved, and thereby the reaction activity is remarkably improved. In addition, since the reaction equilibrium in the production of alcohols largely shifts toward the formation of alcohols in the gas phase, the amounts of esters remaining as the unreacted starting materials and a wax ester formed as the reaction intermediate included in the resulting alcohols at the outlet of the reactor can be expected to be decreased. Actually, however, a side reaction, namely, the reduction reaction of the alcohols to hydrocarbons, is also accelerated, which results in excellent reactivity but poor selectivity, and hence, the yield is rather notably decreased. Furthermore, in the gas phase reaction, the ester used as the starting materials is diluted with a large amount of a hydrogen gas and then fed to a catalytic layer in the form of vapor. As a result, when the amount of the starting materials fed per unit time is made large, namely, when the productivity is to be increased, the flow rate of the vapor within the reactor becomes high to such an extent that the ester can disadvantageously flow through the catalytic layer without being subjected to catalytic reduction reaction. The unreacted starting material ester included in the resulting alcohol has a boiling point region substantially the same as that of the alcohol having a corresponding number of carbon atoms, thereby making it difficult to separate by distillation. In order to overcome this problem, in Japanese Patent Unexamined Publication No. 4-504408, the unreacted starting material ester is first converted to a wax ester in the subsequent process of transesterification, and then the wax ester is separated from the obtained alcohol by distillation. However, such an additional process give rise to higher overall costs.
In the hydrogenation reaction utilizing a gas phase reaction disclosed in Japanese Patent Unexamined Publication No. 4-504408 and Japanese Patent Examined Publication No. 4-57655, since a mixed material of an ester and hydrogen to be brought in contact with a hydrogenation catalyst is always kept at a temperature higher than its dew point, the reaction temperature and pressure, the molar ratio of hydrogen and the kinds of starting materials to be used are considerably restricted. For example, the use of a naturally occurring fatty acid ester, fatty acid triglyceride or fatty acid having short and long chain moieties with 8 to 18 carbon atoms is not practical in view of the necessary amount of hydrogen and the size of equipments.